Radio frequency identification (RFID) systems have conventionally been used in a variety of applications such as product management. Radio frequency tags used in RFID systems may be attached to various products. Proposals have thus been made for antennas suitable for radio frequency tags that exhibit a stable performance in any products to which a radio frequency tag is attached (for example, see Japanese Laid-open Patent Publication Nos. 2011-109552 and 2014-127752).
For example, Japanese Laid-open Patent Publication No. 2011-109552 discloses a loop antenna formed of a first conductor and a second conductor. In this loop antenna, the first conductor forms a first curved surface and includes a third terminal which is disposed at a first end of the first curved surface and coupled to the first terminal in a wireless communication circuit. The first conductor includes a first area at a second end of the first curved surface, the second end being at a side opposite to the first end. The second conductor forms a second curved surface and includes a fourth terminal which is disposed at a third end of the second curved surface and coupled to the second terminal in the wireless communication circuit. The second conductor includes a second area including at a fourth end of the second curved surface, the fourth end being at a side opposite to the third end. The second area is parallel to the first area and the first and second curved surfaces form the loop antenna.
Japanese Laid-open Patent Publication No. 2014-127752 discloses a transmission line with a line having an electrical length of an approximately half-wavelength transmission line that forms an approximately one wavelength transmission line by doubling and strip or linear parallel-line antenna having an approximately quarter-wavelength transmission line that forms an approximately half-wavelength transmission line by doubling.
This linear parallel-line antenna includes a metallic strip, which is disposed on a radiation surface for attaching a feed section or an IC, and a metallic strip, which is disposed below the metallic strip on the radiation surface, in a facing manner. The width of the lower metallic strip is slightly increased to suppress a change in an electrical field of a main transmission line when the antenna is mounted on a metallic surface.